What's your approach to history? Do you like to cleave close to the authentic past or do you mix things up?
In relation to world building, it's kind of a mixed bag. I mean, there are things like 'nation X and nation Y are in a war or territory Z invades territory A which, let's face it, are as generic/commonplace to history as you can get, having been mankind's since history began. And the causes of lots of history are fairly generic also...So-and-so has designs on the throne...this ethnicity (or "race" in D&D) wants to eliminate/enslave that one...nations are competing for resources...or bound by treaties re-enforced with marriages...these things all have plentiful historic examples.
Vague elements that have occurred in RW history but weren't necessarily related. Taking the Middle Ages witch hunts and the Hun invasions to create Orea's historical "Mage Wars"...where non-divine magic-users were sandwiched between a rising intolerant religious power, the Zealotry, and barbarian hordes raiding from the north...generating a mass migration (of those who studied and used "arcana") and necessitating the establishment of the secluded magic-rich realms of R'Hath.
And then there are elements here and there that are direct pulls. Such as the Hungarian Duchess Bathory being a direct inspiration for my world's Countess of Karlith...or the Tuatha de Danaan (their "historic" veracity notwithstanding) being the model for the lords/ruling council of the elvin nation of ShiStaliiri.
Are there certain things you just can't tolerate in a setting?
Mmmm. Hate to directly disagree with your OP, but top of the list without a second thought...firearms. Gunpowder and guns have no place in my fantasy world.
Technology, in general, puts me off whether it's sci-fi elements, like "lazer rifles" that are found and thought of as "magic wands"...or RW stuff like computers, robots...a magical "horseless carriage" that remarkably resembles a Ferrari"? No thanks. I'm playing a "make-believe elves and wizards battling against the encroachment of demons and dragons in a fantasy world" game. Real world stuff like that just ruins my immersion...or at the very least bugs me.
You may have rightly surmised I do not maintain a vacation home in the Barrier Peaks.
Steampunk, in general, gives me a rash.
Being toooooo historically accurate or concerned with such, also a downer most of the time. I'm all for details and internal consistency, but don't get on my case because one society is mostly "early middle ages" so the guys one the other side of the forest can't have plate mail...they just CAN'T! Don't tell me that. That also bugs me.
Guess I just like my vanilla fantasy. I enjoy vanilla. I find it tasty. Some hot fudge or whipped creme every once in a while is fine. But now you want to put some lemon sorbet and strawberry sauce in the same bowl? ARE YOU NUTS!?